Light fixtures are being designed increasingly to include solid-state lighting (SSL), and particularly non-white light-emitting diodes (LEDs), due in large part to the increased efficiency they provide, and at a relatively-high color rendering index (CRI). CRI is a quantitative measure of an ability of light fixture to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source.
With non-white LEDs, significant color mixing is desired to increase CRI and avoid unwanted color spotting from separate color photometric sources. In light fixtures of all types, an optical diffuser is sometimes used to spread out, or scatter, light before the light is emitted from the fixture, resulting in a softer resulting light. Example diffusers include reflectors and lenses.
Using a light fixture employing an indirect configuration, by which light is reflected within the fixture before being emitted by the fixture, is one of the best ways to mix light.
These fixtures include a form of cavity in which light is mixed. The cavity includes, a reflective surface, which can be a diffusive reflector and a light source—e.g., a single-sided light source emitting light toward the reflective surface.
Turning to the figures, and more particularly the first figure, FIG. 1 shows a perspective view of an example reflective, or indirect light or optical fixture 100. The light fixture 100 includes a light engine or source 102, such as an LED board. The engine 102 may include or be connected directly to a heat sink (not shown in detail in FIG. 1). A heat sink is typically required for SSL sources, which are unable to dissipate heat sufficiently on their own.
The light fixture 100 also includes a reflector cover 104 having a reflective surface 106. The fixture 100 may also include a cap or housing 108.
In many fixtures, the light source 102 is positioned is positioned central to two reflector cover portions 104A, 1048, as shown in FIG. 1. In operation, light emitted by the light engine 102 is reflected at the surface(s) 106.
The light source 102 in such indirect lighting fixtures is typically a 0°-180° source, and usually a Lambertian-type light source, whereby the light is emitted generally upward from the source, e.g., between 0°-180°—and so having a vertically upward vector —to impinge on the reflector 104, and reflect from the surface 106 generally downward to outside of the fixture 100.
A common type of indirect lighting fixture is a troffer-type light fixture. While this type of fixture has typically been designed to accommodate standard fluorescent lamps (e.g., T12, T8, T5 format fluorescent lamps), it is being designed increasingly for SSL, including, primarily, LEDs. Troffer light fixtures are the most common lighting fixture used in commercial office spaces, schools, retail, and lab facilities. They are sometimes implemented within a ceiling surface-mount box, but are usually fit mounted about a ceiling recess or plenum that receives portions of the fixture extending above a ceiling plane. Troffers are typically rectangular (elongated or square), but can be round or oval, and are categorized commercially by their size. Example sizes for rectangular troffers include two-by-four (2′ by 4′, or 2×4), two-by-two (2′ by 2′, or 2×2), one-by-four (1′ by 4′, or 1×4), one-by-six (1′ by 6′, or 1×6), and four-by-four (4′ by 4′, or 4×4).
Design goals include providing light of desired strength and quality, such as color—e.g., a soft clear or white light from all angles at which light is emitted from the fixture 100. Traditional indirect lighting fixtures do not provide sufficient color mixing, but are much better than direct and edge-lit light fixtures.
Less than desirable performance of all traditional light fixtures results largely from insufficient diffusion, or insufficient color mixing. Unwanted color separation produces light in which one or more colors are visible undesirably, such as any non-white color being visible when white, or colorless, light output is desired.
The undesirable light characteristics are present commonly in light fixtures using non-white SSLs, such as a blue-shifted-yellow plus red (BSY+R) LED. For some traditional fixtures, undesirable light characteristics have been found present especially in connection with rays arriving at an intra-fixture diffuser at relatively high incident angles.